Field of the Invention
The present invention relates to a ghost cancelling method and an apparatus thereof for a television video signal. More particularly, the present invention relates to a ghost cancelling method and an apparatus thereof for automatically cancelling a ghost by inputting an original video signal to a transversal filter to generate a ghost correction signal and mixing the ghost correction signal with the original video signal.
When a television signal is transmitted over a channel, the television signal may pick up a ghost signal as a result of a multiplexed path reflected from a big object such as a building, etc. Such a ghost signal is received at a time that is delayed in relation to the main signal, due to the added length of the ghost signal path. Also, since the ghost signal has a narrower amplitude than does the main signal, it appears as a dark image which is offset from the image formed by the main signal on a television screen, when the television signal is reproduced by a receiver.
To remove such an undesired ghost signal from the received signal, various prior art technologies have been proposed. These prior art technologies utilize a common principle in which the main signal is delayed so as to be synchronized with the ghost signal, and the amplitude of the main signal is attenuated so as to be equal to that of the ghost signal.
As a result, the attenuated and delayed main signal, that is, the ghost correction signal, cancels the ghost signal.
As shown in FIG. 1, showing a block diagram of a conventional ghost cancelling apparatus, the above-mentioned ghost correction signal is generated by a transversal filter 3. The ghost correction signal is subtracted from the input image signal x(n) in a subtracter 2, thereby generating a ghost-cancelled signal y(n). The transversal filter 3 comprises a tap delay line 4, a coefficient circuit 5, a filter coefficient memory 8 and an adder 6. The tap delay line 4 sequentially delays the output signal y(n) from the subtracter 2 every cycle. The coefficient circuit 5, which is composed of a plurality of multipliers, each of which is connected to each tap corresponding to each delayed output, multiplies each output by a filter coefficient that is supplied to the coefficient circuit 5 from a filter coefficient memory 8, and supplies each multiplied result to an adder 6. The output of the adder 6 is the ghost correction signal supplied to the subtracter 2.
The filter coefficient of each tap is corrected in a filter coefficient corrector 11, based on an error signal e(n) between a ghost cancelling reference signal r(n) and the ghost-cancelled output signal y(n). The correction occurs every cycle, to thereby generate the optimum ghost correction signal.
In such a conventional ghost cancelling system, an initial filter coefficient of each tap is set to "0". Then, the filter coefficient is corrected based on the video signal generated due to this initial filter coefficient and the ghost-cancelled reference signal. By repeating the above-described filter coefficient correction process, the optimum filter coefficient can be obtained. Accordingly, since a number of repeated correction processes to reach the optimum filter coefficient are required, it takes a long time to cancel the ghost signal. For example, it can take three seconds or so to cancel it. Thus, since the conventional system is delayed in its response to the generated ghost, the ghost is often not sufficiently cancelled. In particular, in cases of time-variant moving ghosts generated due to reflected signals from an airplane, it is not possible to cancel the ghosts adaptively and continuously.